1. Field of the Invention
The present invention relates to a method of calculating a write condition detection index, the method determining an index used for detection of a write condition for an optical disk when information is written to the optical disk, as well as an optical disk writing method and apparatus using the above method.
2. Description of the Related Art
Parameters indicating a write condition for an optical disk include an “asymmetry value”, indicating the degree of asymmetry of a RF (Radio Frequency) waveform obtained by reading a written waveform, and a “β value (β)”.
If a data signal is to be written to an optical disk such as a CD-R, it is modulated using an EFM (Eight to Fourteen Modulation) method. With this method, nine time intervals that are each three to eleven times (other values may be used depending on the type of the disk) as long as a predetermined reference time interval T are provided as high- and low-level time intervals for a reference digital signal. FIGS. 8A, 8B and 8C show waveforms read from a disk to which information has been written in the above manner. FIG. 8A indicates that the amplitude has a large value when lands are long and that the amplitude is low when the lands have a length of 3T. Further, as shown in FIG. 8A, if the ground of a non-DC-cut (DC-coupled) HF signal is defined as a 0 level position, and the land and pit levels at 3T and the land and pit levels at 11T are defined as I3L and I3P, and I11L and I11P, respectively, then the asymmetry is expressed by the following equation.Asymmetry value={(I3L+I3P)/2−(I11L+I11P)/2}/(I11L−I11P)  (6)
As shown in FIGS. 8A, 8B and 8C, the asymmetry value decreases with increasing write power, while increasing with decreasing write power.
The β value is the index that indicates the degree of asymmetry of a waveform written to an optical disk. For the RF waveform shown in FIG. 8C and obtained by reading the written waveform, if the AC-ground of a DC-cut (AC-coupled) HF signal is defined as a 0 level and the land- and pit-side levels of the HF signal envelope are defined as A1 and A2, then the β is expressed by the following equation:β value=(A1+A2)/(A1−A2)  (7)
As shown in FIGS. 8A, 8B and 8C, in contrast to the asymmetry value, the β value increases consistently with write power, while decreasing consistently with write power.
The β value (or asymmetry value. These will be interchangeably used below) and jitters have the relationship shown in FIG. 9. If the β value is excessively large or small, jitters are likely to become worse. The range of the β value corresponding to tolerable jitter values is generally called a “power margin”. The power margin varies depending on the type of the optical disk. In particular, optical disks with narrow power margins have their β value vary significantly, and thus need to have such a write condition that the β value is stable and uniform all over the surface of the optical disk. In recent years, a high-double-speed write technique has been popularized, and such a technique tends to involve a much narrower power margin. Thus, it has recently been more desirable to establish such a write condition that the β value is stable and uniform all over the surface of the optical disk.
Under these circumstances, a normal method of writing information to an optical disk comprises calibrating optimum write power (this operation will hereinafter be referred to as “OPC”) in a predetermined power calibration area (this will hereinafter be referred to as a “PCA”) and then actually writing information to a data write area with the fixed optimum write power determined by OPC.
However, due to the causes listed below, the write condition for the optical disk may not be optimum in spite of the optimum power determined by the OPC.                1) Variation in the characteristics of the optical disk depending on a position within the surface of the disk.        2) Variation in the offset between the optical axis of a laser beam from an optical head and the mechanical inclination of the write surface of the optical disk, depending on a position in the surface of the disk. This is due to a variation in radial skew within the surface, the warp of the disk, or the like. The term “radial skew” refers to the angle between the optical axis of the optical pickup in its radial direction and the normal of the optical disk.        3) Variation in the characteristics of the optical disk caused by a variation in temperature between OPC and actual writes.        4) Variation in write characteristics caused by a variation in the wavelength of a semiconductor laser element associated with a variation in temperature between OPC and actual writes.        
To solve these problems, an operation may be performed which comprises detecting the write condition during a write using various methods, correcting the write power on the basis of the detected conditions, and maintaining the write condition established during the OPC (this operation is generally referred to as “running OPC”).
For example, the techniques disclosed in Japanese Patent Application Laid-Open No. 2000-215454 (hereinafter referred to as a “first conventional example”), Japanese Patent Application Laid-Open No. 9-270128 (hereinafter referred to as a “second conventional example”), and Japanese Patent Application Laid-Open No. 9-91705 (hereinafter referred to as a “third conventional example”) are known as running OPC.
In the first conventional example, on the basis of the peak value of the level of write pit reflected light (this will hereinafter be referred to as a “peak value”) and the sampling hold level of the latter half of the write pit reflected light (this will hereinafter be referred to as a “B value”), the write condition is detected using the following equation, to correct the write power:Write condition detection index=(B value)/(peak value)  (11)
That is, during OPC, the write condition detection index is measured on the basis of the above equation and used as a write condition detection index target value. Further, the write power is controlled so that the write condition detection index measured during a write to a data write area equals the target value.
In the second conventional example, on the basis of the peak value and the B value, the write condition is detected using the following equation, to correct the write power:Write condition detection index=(peak value)/N−(B value)  (12)where N is an experimental value determined for each type of optical disk.
In the third conventional example, the peak value is not used as an operational parameter in contrast to the first and second conventional examples. That is, on the basis of the level of write pit reflected light and the write power, the write condition is detected using the following equation, to correct the write power:Write condition detection index=(write pit reflected light level)/(write power)  (13)
That is, the write condition detection index is measured at the start of a write to a data write area on the basis of the above equation and used as a write condition detection index target value. Further, the write power is controlled so that the write condition detection indices measured during subsequent writes to data write areas equal the target value.
However, the first to third conventional examples have the following problems:
In the first and second conventional examples, the peak value included in both Equations (11) and (12) is drastically affected by the frequency characteristics of a reflected light detecting circuit, and the frequency characteristics vary even with temperature. Thus, it is difficult to stably measure the peak value. Further, a peak hold circuit is required to obtain a peak value, thereby increasing circuit costs. Furthermore, due to various factors, a variation in write condition cannot be properly followed by using the method of controlling the write power on the basis of the write condition detection index obtained using Equations (11) and (12). Furthermore, for certain types of optical disks, the write condition detection index cannot be properly detected.
In the second conventional example, the value N included in Equation (12) is given for each type of optical disk, so that this method cannot deal with many types of optical disks easily. Furthermore, different optical write devices have different Ns, so that this example is not suited for mass production.
In the third conventional example, the write condition detection index determined using Equation (13) exhibits a small variation with respect to the write power for some types of optical disks and thus has low detection sensitivity. Accordingly, it is difficult to increase the accuracy of running OPC. Further, due to various factors, a variation in write condition cannot be properly followed by using the method of controlling the write power on the basis of the write condition detection index obtained using Equation (13).
FIG. 10 is a graph showing the results of running OPC according to the third conventional example. If information is written to the entire surface of the optical disk with the write power fixed, that is, without running OPC, the β value is about 10% smaller at the outer circumference of the disk than at the inner circumference thereof. If running OPC is executed on this optical disk to write information to the entire surface, the β value is about 5% larger at the outer circumference of the disk than at the inner circumference thereof. The results of the experiments indicate that running OPC improved the variation in β value from −10% only to +5%. However, the ideal variation is 0%.